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llvm-project/lld/ELF/Writer.cpp

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//===- Writer.cpp ---------------------------------------------------------===//
//
// The LLVM Linker
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
#include "Writer.h"
#include "Config.h"
#include "OutputSections.h"
#include "SymbolTable.h"
#include "Target.h"
#include "llvm/Support/FileOutputBuffer.h"
using namespace llvm;
using namespace llvm::ELF;
using namespace llvm::object;
using namespace lld;
using namespace lld::elf2;
static const int PageSize = 4096;
// On freebsd x86_64 the first page cannot be mmaped.
// On linux that is controled by vm.mmap_min_addr. At least on some x86_64
// installs that is 65536, so the first 15 pages cannot be used.
// Given that, the smallest value that can be used in here is 0x10000.
// If using 2MB pages, the smallest page aligned address that works is
// 0x200000, but it looks like every OS uses 4k pages for executables.
// FIXME: This is architecture and OS dependent.
static const int VAStart = 0x10000;
namespace {
static uint32_t toPHDRFlags(uint64_t Flags) {
uint32_t Ret = PF_R;
if (Flags & SHF_WRITE)
Ret |= PF_W;
if (Flags & SHF_EXECINSTR)
Ret |= PF_X;
return Ret;
}
template <class ELFT> struct ProgramHeader {
typedef typename ELFFile<ELFT>::uintX_t uintX_t;
typedef typename ELFFile<ELFT>::Elf_Phdr Elf_Phdr;
ProgramHeader(uintX_t Type, uintX_t Flags, uintX_t FileOff, uintX_t VA) {
std::memset(&Header, 0, sizeof(Elf_Phdr));
Header.p_type = Type;
Header.p_flags = Flags;
Header.p_align = PageSize;
Header.p_offset = FileOff;
Header.p_vaddr = VA;
Header.p_paddr = VA;
}
void setValuesFromSection(OutputSectionBase<ELFT::Is64Bits> &Sec) {
Header.p_flags = toPHDRFlags(Sec.getFlags());
Header.p_offset = Sec.getFileOff();
Header.p_vaddr = Sec.getVA();
Header.p_paddr = Header.p_vaddr;
Header.p_filesz = Sec.getSize();
Header.p_memsz = Header.p_filesz;
Header.p_align = Sec.getAlign();
}
Elf_Phdr Header;
bool Closed = false;
};
// The writer writes a SymbolTable result to a file.
template <class ELFT> class Writer {
public:
typedef typename ELFFile<ELFT>::uintX_t uintX_t;
typedef typename ELFFile<ELFT>::Elf_Shdr Elf_Shdr;
typedef typename ELFFile<ELFT>::Elf_Ehdr Elf_Ehdr;
typedef typename ELFFile<ELFT>::Elf_Phdr Elf_Phdr;
typedef typename ELFFile<ELFT>::Elf_Sym Elf_Sym;
typedef typename ELFFile<ELFT>::Elf_Sym_Range Elf_Sym_Range;
typedef typename ELFFile<ELFT>::Elf_Rela Elf_Rela;
Writer(SymbolTable *T)
: SymTabSec(*T, StrTabSec, BssSec), DynSymSec(*T, DynStrSec, BssSec),
RelaDynSec(DynSymSec, GotSec, T->shouldUseRela()), PltSec(GotSec),
HashSec(DynSymSec), DynamicSec(*T, HashSec, RelaDynSec, BssSec),
BssSec(PltSec, GotSec, BssSec, ".bss", SHT_NOBITS,
SHF_ALLOC | SHF_WRITE) {}
void run();
private:
void createSections();
template <bool isRela>
void scanRelocs(const InputSection<ELFT> &C,
iterator_range<const Elf_Rel_Impl<ELFT, isRela> *> Rels);
void scanRelocs(const InputSection<ELFT> &C);
void assignAddresses();
void openFile(StringRef OutputPath);
void writeHeader();
void writeSections();
bool needsInterpSection() const {
return !SymTabSec.getSymTable().getSharedFiles().empty() &&
!Config->DynamicLinker.empty();
}
bool needsDynamicSections() const {
return !SymTabSec.getSymTable().getSharedFiles().empty() || Config->Shared;
}
unsigned getVAStart() const { return Config->Shared ? 0 : VAStart; }
std::unique_ptr<llvm::FileOutputBuffer> Buffer;
llvm::SpecificBumpPtrAllocator<OutputSection<ELFT>> CAlloc;
std::vector<OutputSectionBase<ELFT::Is64Bits> *> OutputSections;
unsigned getNumSections() const { return OutputSections.size() + 1; }
llvm::BumpPtrAllocator PAlloc;
std::vector<ProgramHeader<ELFT> *> PHDRs;
ProgramHeader<ELFT> FileHeaderPHDR{PT_LOAD, PF_R, 0, 0};
ProgramHeader<ELFT> InterpPHDR{PT_INTERP, 0, 0, 0};
ProgramHeader<ELFT> DynamicPHDR{PT_DYNAMIC, 0, 0, 0};
uintX_t FileSize;
uintX_t ProgramHeaderOff;
uintX_t SectionHeaderOff;
StringTableSection<ELFT::Is64Bits> StrTabSec = { /*dynamic=*/false };
StringTableSection<ELFT::Is64Bits> DynStrSec = { /*dynamic=*/true };
lld::elf2::SymbolTableSection<ELFT> SymTabSec;
lld::elf2::SymbolTableSection<ELFT> DynSymSec;
RelocationSection<ELFT> RelaDynSec;
GotSection<ELFT> GotSec;
PltSection<ELFT> PltSec;
HashTableSection<ELFT> HashSec;
DynamicSection<ELFT> DynamicSec;
InterpSection<ELFT::Is64Bits> InterpSec;
OutputSection<ELFT> BssSec;
};
} // anonymous namespace
namespace lld {
namespace elf2 {
template <class ELFT>
void writeResult(SymbolTable *Symtab) { Writer<ELFT>(Symtab).run(); }
template void writeResult<ELF32LE>(SymbolTable *);
template void writeResult<ELF32BE>(SymbolTable *);
template void writeResult<ELF64LE>(SymbolTable *);
template void writeResult<ELF64BE>(SymbolTable *);
} // namespace elf2
} // namespace lld
// The main function of the writer.
template <class ELFT> void Writer<ELFT>::run() {
createSections();
assignAddresses();
openFile(Config->OutputFile);
writeHeader();
writeSections();
error(Buffer->commit());
}
namespace {
template <bool Is64Bits> struct SectionKey {
typedef typename std::conditional<Is64Bits, uint64_t, uint32_t>::type uintX_t;
StringRef Name;
uint32_t Type;
uintX_t Flags;
};
}
namespace llvm {
template <bool Is64Bits> struct DenseMapInfo<SectionKey<Is64Bits>> {
static SectionKey<Is64Bits> getEmptyKey() {
return SectionKey<Is64Bits>{DenseMapInfo<StringRef>::getEmptyKey(), 0, 0};
}
static SectionKey<Is64Bits> getTombstoneKey() {
return SectionKey<Is64Bits>{DenseMapInfo<StringRef>::getTombstoneKey(), 0,
0};
}
static unsigned getHashValue(const SectionKey<Is64Bits> &Val) {
return hash_combine(Val.Name, Val.Type, Val.Flags);
}
static bool isEqual(const SectionKey<Is64Bits> &LHS,
const SectionKey<Is64Bits> &RHS) {
return DenseMapInfo<StringRef>::isEqual(LHS.Name, RHS.Name) &&
LHS.Type == RHS.Type && LHS.Flags == RHS.Flags;
}
};
}
// The reason we have to do this early scan is as follows
// * To mmap the output file, we need to know the size
// * For that, we need to know how many dynamic relocs we will have.
// It might be possible to avoid this by outputting the file with write:
// * Write the allocated output sections, computing addresses.
// * Apply relocations, recording which ones require a dynamic reloc.
// * Write the dynamic relocations.
// * Write the rest of the file.
template <class ELFT>
template <bool isRela>
void Writer<ELFT>::scanRelocs(
const InputSection<ELFT> &C,
iterator_range<const Elf_Rel_Impl<ELFT, isRela> *> Rels) {
typedef Elf_Rel_Impl<ELFT, isRela> RelType;
const ObjectFile<ELFT> &File = *C.getFile();
bool IsMips64EL = File.getObj().isMips64EL();
for (const RelType &RI : Rels) {
uint32_t SymIndex = RI.getSymbol(IsMips64EL);
SymbolBody *Body = File.getSymbolBody(SymIndex);
if (!Body)
continue;
uint32_t Type = RI.getType(IsMips64EL);
if (Target->relocNeedsPlt(Type, *Body)) {
if (Body->isInPlt())
continue;
PltSec.addEntry(Body);
}
if (Target->relocNeedsGot(Type, *Body)) {
if (Body->isInGot())
continue;
GotSec.addEntry(Body);
} else if (!isa<SharedSymbol<ELFT>>(Body))
continue;
Body->setUsedInDynamicReloc();
RelaDynSec.addReloc({C, RI});
}
}
template <class ELFT>
void Writer<ELFT>::scanRelocs(const InputSection<ELFT> &C) {
ObjectFile<ELFT> *File = C.getFile();
ELFFile<ELFT> &EObj = File->getObj();
if (!(C.getSectionHdr()->sh_flags & SHF_ALLOC))
return;
for (const Elf_Shdr *RelSec : C.RelocSections) {
if (RelSec->sh_type == SHT_RELA)
scanRelocs(C, EObj.relas(RelSec));
else
scanRelocs(C, EObj.rels(RelSec));
}
}
template <class ELFT>
static void reportUndefined(const SymbolTable &S, const SymbolBody &Sym) {
typedef typename ELFFile<ELFT>::Elf_Sym Elf_Sym;
typedef typename ELFFile<ELFT>::Elf_Sym_Range Elf_Sym_Range;
if (Config->Shared && !Config->NoUndefined)
return;
const Elf_Sym &SymE = cast<ELFSymbolBody<ELFT>>(Sym).Sym;
ELFFileBase *SymFile = nullptr;
for (const std::unique_ptr<ObjectFileBase> &F : S.getObjectFiles()) {
const auto &File = cast<ObjectFile<ELFT>>(*F);
Elf_Sym_Range Syms = File.getObj().symbols(File.getSymbolTable());
if (&SymE > Syms.begin() && &SymE < Syms.end())
SymFile = F.get();
}
std::string Message = "undefined symbol: " + Sym.getName().str();
if (SymFile)
Message += " in " + SymFile->getName().str();
if (Config->NoInhibitExec)
warning(Message);
else
error(Message);
}
// Create output section objects and add them to OutputSections.
template <class ELFT> void Writer<ELFT>::createSections() {
SmallDenseMap<SectionKey<ELFT::Is64Bits>, OutputSection<ELFT> *> Map;
OutputSections.push_back(&BssSec);
Map[{BssSec.getName(), BssSec.getType(), BssSec.getFlags()}] = &BssSec;
SymbolTable &Symtab = SymTabSec.getSymTable();
for (const std::unique_ptr<ObjectFileBase> &FileB : Symtab.getObjectFiles()) {
auto &File = cast<ObjectFile<ELFT>>(*FileB);
if (!Config->DiscardAll) {
Elf_Sym_Range Syms = File.getLocalSymbols();
for (const Elf_Sym &Sym : Syms) {
ErrorOr<StringRef> SymName = Sym.getName(File.getStringTable());
if (SymName && shouldKeepInSymtab(*SymName))
SymTabSec.addSymbol(*SymName, true);
}
}
for (InputSection<ELFT> *C : File.getSections()) {
if (!C)
continue;
const Elf_Shdr *H = C->getSectionHdr();
SectionKey<ELFT::Is64Bits> Key{C->getSectionName(), H->sh_type,
H->sh_flags};
OutputSection<ELFT> *&Sec = Map[Key];
if (!Sec) {
Sec = new (CAlloc.Allocate()) OutputSection<ELFT>(
PltSec, GotSec, BssSec, Key.Name, Key.Type, Key.Flags);
OutputSections.push_back(Sec);
}
Sec->addSection(C);
scanRelocs(*C);
}
}
DynamicSec.PreInitArraySec =
Map.lookup({".preinit_array", SHT_PREINIT_ARRAY, SHF_WRITE | SHF_ALLOC});
DynamicSec.InitArraySec =
Map.lookup({".init_array", SHT_INIT_ARRAY, SHF_WRITE | SHF_ALLOC});
DynamicSec.FiniArraySec =
Map.lookup({".fini_array", SHT_FINI_ARRAY, SHF_WRITE | SHF_ALLOC});
if (OutputSection<ELFT> *OS = DynamicSec.InitArraySec) {
Symtab.addSyntheticSym<ELFT>("__init_array_start", *OS, 0);
Symtab.addSyntheticSym<ELFT>("__init_array_end", *OS, OS->getSize());
} else {
Symtab.addIgnoredSym<ELFT>("__init_array_start");
Symtab.addIgnoredSym<ELFT>("__init_array_end");
}
// FIXME: Try to avoid the extra walk over all global symbols.
std::vector<DefinedCommon<ELFT> *> CommonSymbols;
for (auto &P : Symtab.getSymbols()) {
StringRef Name = P.first;
SymbolBody *Body = P.second->Body;
if (auto *U = dyn_cast<Undefined<ELFT>>(Body)) {
if (!U->isWeak() && !U->canKeepUndefined())
reportUndefined<ELFT>(Symtab, *Body);
}
if (auto *C = dyn_cast<DefinedCommon<ELFT>>(Body))
CommonSymbols.push_back(C);
if (!includeInSymtab(*Body))
continue;
SymTabSec.addSymbol(Name);
if (needsDynamicSections() && includeInDynamicSymtab(*Body))
HashSec.addSymbol(Body);
}
// Sort the common symbols by alignment as an heuristic to pack them better.
std::stable_sort(
CommonSymbols.begin(), CommonSymbols.end(),
[](const DefinedCommon<ELFT> *A, const DefinedCommon<ELFT> *B) {
return A->MaxAlignment > B->MaxAlignment;
});
uintX_t Off = BssSec.getSize();
for (DefinedCommon<ELFT> *C : CommonSymbols) {
const Elf_Sym &Sym = C->Sym;
uintX_t Align = C->MaxAlignment;
Off = RoundUpToAlignment(Off, Align);
C->OffsetInBSS = Off;
Off += Sym.st_size;
}
BssSec.setSize(Off);
OutputSections.push_back(&SymTabSec);
if (needsDynamicSections()) {
if (needsInterpSection())
OutputSections.push_back(&InterpSec);
OutputSections.push_back(&DynSymSec);
OutputSections.push_back(&HashSec);
OutputSections.push_back(&DynamicSec);
OutputSections.push_back(&DynStrSec);
if (RelaDynSec.hasRelocs())
OutputSections.push_back(&RelaDynSec);
}
if (!GotSec.empty())
OutputSections.push_back(&GotSec);
if (!PltSec.empty())
OutputSections.push_back(&PltSec);
std::stable_sort(
OutputSections.begin(), OutputSections.end(),
[](OutputSectionBase<ELFT::Is64Bits> *A,
OutputSectionBase<ELFT::Is64Bits> *B) {
uintX_t AFlags = A->getFlags();
uintX_t BFlags = B->getFlags();
// Allocatable sections go first to reduce the total PT_LOAD size and
// so debug info doesn't change addresses in actual code.
bool AIsAlloc = AFlags & SHF_ALLOC;
bool BIsAlloc = BFlags & SHF_ALLOC;
if (AIsAlloc != BIsAlloc)
return AIsAlloc;
// We don't have any special requirements for the relative order of
// two non allocatable sections.
if (!AIsAlloc)
return false;
// We want the read only sections first so that they go in the PT_LOAD
// covering the program headers at the start of the file.
bool AIsWritable = AFlags & SHF_WRITE;
bool BIsWritable = BFlags & SHF_WRITE;
if (AIsWritable != BIsWritable)
return BIsWritable;
// For a corresponding reason, put non exec sections first (the program
// header PT_LOAD is not executable).
bool AIsExec = AFlags & SHF_EXECINSTR;
bool BIsExec = BFlags & SHF_EXECINSTR;
if (AIsExec != BIsExec)
return BIsExec;
// If we got here we know that both A and B and in the same PT_LOAD.
// The last requirement we have is to put nobits section last. The
// reason is that the only thing the dynamic linker will see about
// them is a p_memsz that is larger than p_filesz. Seeing that it
// zeros the end of the PT_LOAD, so that has to correspond to the
// nobits sections.
return A->getType() != SHT_NOBITS && B->getType() == SHT_NOBITS;
});
// Always put StrTabSec last so that no section names are added to it after
// it's finalized.
OutputSections.push_back(&StrTabSec);
for (unsigned I = 0, N = OutputSections.size(); I < N; ++I)
OutputSections[I]->setSectionIndex(I + 1);
// Fill the DynStrSec early.
DynamicSec.finalize();
}
template <class ELFT>
static bool needsPHDR(OutputSectionBase<ELFT::Is64Bits> *Sec) {
return Sec->getFlags() & SHF_ALLOC;
}
// Visits all sections to assign incremental, non-overlapping RVAs and
// file offsets.
template <class ELFT> void Writer<ELFT>::assignAddresses() {
assert(!OutputSections.empty() && "No output sections to layout!");
uintX_t VA = getVAStart();
uintX_t FileOff = 0;
FileOff += sizeof(Elf_Ehdr);
VA += sizeof(Elf_Ehdr);
// Reserve space for PHDRs.
ProgramHeaderOff = FileOff;
FileOff = RoundUpToAlignment(FileOff, PageSize);
VA = RoundUpToAlignment(VA, PageSize);
if (needsInterpSection())
PHDRs.push_back(&InterpPHDR);
// Create a PHDR for the file header.
PHDRs.push_back(&FileHeaderPHDR);
FileHeaderPHDR.Header.p_vaddr = getVAStart();
FileHeaderPHDR.Header.p_paddr = getVAStart();
FileHeaderPHDR.Header.p_align = PageSize;
for (OutputSectionBase<ELFT::Is64Bits> *Sec : OutputSections) {
StrTabSec.add(Sec->getName());
Sec->finalize();
if (Sec->getSize()) {
uintX_t Flags = toPHDRFlags(Sec->getFlags());
ProgramHeader<ELFT> *Last = PHDRs.back();
if (Last->Header.p_flags != Flags || !needsPHDR<ELFT>(Sec)) {
// Flags changed. End current PHDR and potentially create a new one.
if (!Last->Closed) {
Last->Header.p_filesz = FileOff - Last->Header.p_offset;
Last->Header.p_memsz = VA - Last->Header.p_vaddr;
Last->Closed = true;
}
if (needsPHDR<ELFT>(Sec)) {
VA = RoundUpToAlignment(VA, PageSize);
FileOff = RoundUpToAlignment(FileOff, PageSize);
PHDRs.push_back(new (PAlloc)
ProgramHeader<ELFT>(PT_LOAD, Flags, FileOff, VA));
}
}
}
uintX_t Align = Sec->getAlign();
uintX_t Size = Sec->getSize();
if (Sec->getFlags() & SHF_ALLOC) {
VA = RoundUpToAlignment(VA, Align);
Sec->setVA(VA);
VA += Size;
}
FileOff = RoundUpToAlignment(FileOff, Align);
Sec->setFileOffset(FileOff);
if (Sec->getType() != SHT_NOBITS)
FileOff += Size;
}
// Add a PHDR for the dynamic table.
if (needsDynamicSections())
PHDRs.push_back(&DynamicPHDR);
FileOff += OffsetToAlignment(FileOff, ELFT::Is64Bits ? 8 : 4);
// Add space for section headers.
SectionHeaderOff = FileOff;
FileOff += getNumSections() * sizeof(Elf_Shdr);
FileSize = FileOff;
}
template <class ELFT> void Writer<ELFT>::writeHeader() {
uint8_t *Buf = Buffer->getBufferStart();
auto *EHdr = reinterpret_cast<Elf_Ehdr *>(Buf);
EHdr->e_ident[EI_MAG0] = 0x7F;
EHdr->e_ident[EI_MAG1] = 0x45;
EHdr->e_ident[EI_MAG2] = 0x4C;
EHdr->e_ident[EI_MAG3] = 0x46;
EHdr->e_ident[EI_CLASS] = ELFT::Is64Bits ? ELFCLASS64 : ELFCLASS32;
EHdr->e_ident[EI_DATA] = ELFT::TargetEndianness == llvm::support::little
? ELFDATA2LSB
: ELFDATA2MSB;
EHdr->e_ident[EI_VERSION] = EV_CURRENT;
const SymbolTable &Symtab = SymTabSec.getSymTable();
auto &FirstObj = cast<ObjectFile<ELFT>>(*Symtab.getFirstELF());
EHdr->e_ident[EI_OSABI] = FirstObj.getOSABI();
// FIXME: Generalize the segment construction similar to how we create
// output sections.
EHdr->e_type = Config->Shared ? ET_DYN : ET_EXEC;
EHdr->e_machine = FirstObj.getEMachine();
EHdr->e_version = EV_CURRENT;
SymbolBody *Entry = Symtab.getEntrySym();
EHdr->e_entry =
Entry ? getSymVA(cast<ELFSymbolBody<ELFT>>(*Entry), BssSec) : 0;
EHdr->e_phoff = ProgramHeaderOff;
EHdr->e_shoff = SectionHeaderOff;
EHdr->e_ehsize = sizeof(Elf_Ehdr);
EHdr->e_phentsize = sizeof(Elf_Phdr);
EHdr->e_phnum = PHDRs.size();
EHdr->e_shentsize = sizeof(Elf_Shdr);
EHdr->e_shnum = getNumSections();
EHdr->e_shstrndx = StrTabSec.getSectionIndex();
// If nothing was merged into the file header PT_LOAD, set the size correctly.
if (FileHeaderPHDR.Header.p_filesz == PageSize) {
uint64_t Size = sizeof(Elf_Ehdr) + sizeof(Elf_Phdr) * PHDRs.size();
FileHeaderPHDR.Header.p_filesz = Size;
FileHeaderPHDR.Header.p_memsz = Size;
}
if (needsInterpSection())
InterpPHDR.setValuesFromSection(InterpSec);
if (needsDynamicSections())
DynamicPHDR.setValuesFromSection(DynamicSec);
auto PHdrs = reinterpret_cast<Elf_Phdr *>(Buf + EHdr->e_phoff);
for (ProgramHeader<ELFT> *PHDR : PHDRs)
*PHdrs++ = PHDR->Header;
auto SHdrs = reinterpret_cast<Elf_Shdr *>(Buf + EHdr->e_shoff);
// First entry is null.
++SHdrs;
for (OutputSectionBase<ELFT::Is64Bits> *Sec : OutputSections) {
Sec->setNameOffset(StrTabSec.getFileOff(Sec->getName()));
Sec->template writeHeaderTo<ELFT::TargetEndianness>(SHdrs++);
}
}
template <class ELFT> void Writer<ELFT>::openFile(StringRef Path) {
ErrorOr<std::unique_ptr<FileOutputBuffer>> BufferOrErr =
FileOutputBuffer::create(Path, FileSize, FileOutputBuffer::F_executable);
error(BufferOrErr, Twine("failed to open ") + Path);
Buffer = std::move(*BufferOrErr);
}
// Write section contents to a mmap'ed file.
template <class ELFT> void Writer<ELFT>::writeSections() {
uint8_t *Buf = Buffer->getBufferStart();
for (OutputSectionBase<ELFT::Is64Bits> *Sec : OutputSections)
Sec->writeTo(Buf + Sec->getFileOff());
}
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